Develop Reference

Android custom Intent class

I want to send a complex object from one Activity to another, so this complex object cannot be serialize, because some fields in this object are system classes and natively non-serializable. That's why I want to create my custom class named "MyIntent" inherits from android.content.Intent, with this way, I can put any type of objects in my custom Intent. But, in the second activity, getIntent() method returns android.content.Intent instance, not MyIntent instance. I don't understand why, maybe i did not implement android.content.Intent class correctly.
public class MyIntent extends Intent
{
}
//starting activity
MyIntent intent = new MyIntent();
intent.setClass(context, SecondActivity.class);
context.startActivity(intent);
//inside of onCreate() of SecondActivity
Intent i = getIntent();
i instanceof MyIntent = false

startActivity() involves inter-process communication, even when the activity you are starting is in the same app and the same process. Android will ignore your subclass, because the OS process that handles startActivity() requests cannot use your subclass, as that is in your app, and the OS process is not your app. And, the IPC that delivers the request to start the activity back to your process will not know anything about your subclass.
Either:
Combine these into one activity (e.g., using two fragments), or
Use some sort of singleton data manager that both activities can work with

use function from main activity in another java file

I’m trying to call a method in my main Activity from another file, but it crashes. I can easily do this the other way. Can you not invoke methods of the main Activity from another Activity?
public class MainActivity extends Activity {
….
……..
……
public void my_function(String a){
//do some stuff
}
}
package main;
public class stuff extends Activity {
….
….
…
MainActivity run = new MainActivity();
run.my_function(String a);
}
}

If you want a method to get called from different classes, create a seperate class
and encapsulate the logic. See an activity more as a view controller, the logic
should be somewhere else.
Sorry for my english, hope this helps.
Steve

The problem is this line
MainActivity run = new MainActivity();
you never instantiate Activities directly, you need to instantiate them through Intent objects.
Intent intent = new Intent(this, MainActivity.class);
If you want to instantiate a new activity use an Intent. If you want to call my_function in an activity that is already instantiated, you need first to get a pointer to that Activity, which in Android is a little bit tricky.
Additionally, the fact that you want to call a method from another activity is a sign of bad design, or that you don't fully understand the Android way of doing things.
Think if there's any other approach you can use for your purpose.

Another way that you can exchange data between activities

What is different between static field and other ways in store data when the application is run?
I'm using static fields for pass data between activities and worked perfectly:
I defined this class :
public class Info
{
public static int ID = 0;
public static String NAME = "TEST";
public static TestClass testclass = null;
}
and I can store my data anywhere:
Info.ID = 5;
Info.NAME = "USER!";
Info.testclass = new TestClass();
and I can get my data anywhere:
Info.ID
Info.NAME
Info.testclass

It is usual to pass data between activities using extras within the intent. Such data persists for the lifetime of the receiving activity (when finished with, the garbage collector can free up the memory).
Or you can store values using SharedPreferences. These will persist between sessions and are stored as key/value pairs in a file (so don't impact memory use as such).
Or you can hold values in static fields (as you are doing here) which persist for the lifetime of your application session. However there is a significant risk with this in that the garbage collector cannot free up memory that is referenced by such fields unless you set them to null when you no longer need the reference. You should never store a reference to an activity/context/view in a static field since you'll leak the entire activity which can amout to a significant amount of memory usage.
http://android-developers.blogspot.fr/2009/01/avoiding-memory-leaks.html
You can pass a class instance within an intent if it is Serializable, e.g.:
Intent intent = new Intent(this, whatever.class);
Bundle b = new Bundle();
b.putSerializable("data", my_object);
intent.putExtras(b);
startActivity(intent);
And in the receiving activity, cast it back to whatever class your object is:
Bundle b = getIntent().getExtras();
my_object = (whatever class is it) b.getSerializable("data");
Many java classes implement Serializable and it is very simple to make your own classes serializable too.

If you're changing activities I'm assuming you're using intents. What you can do is send data with the intent with myIntent.putExtra("some string",someString);.
Then you can receive the info in your new activity using
Intent intent = getIntent();
String someString = intent.getExtra("some string");

You can use intents for passing data between activities.
Your first Activity.java
public void onClick(View v)
{
Intent timer = new Intent (FirstActivity.this,SecondActivity.class);
timer.putExtra("beefType", 5000);
startActivity(timer);
}
Then in your SecondActivity.java file do:
nt beefType = getIntent().getIntExtra("beefType", -1);

You want to share data between activities,you can use intent or shared prefernce.
The difference in using these tow and static data is that,intent and shared prefrence ,at some static data can be empty or null.but sending data using above two methods gurantees that you will get data in next activity ,unless you forcefully remove preference
you can refer this link for more info Static class in Java (Android) - use or not use

There is something called an Application Class in android. Its like a global singleton.
In other words, that Application Class will be common for that entire application.
Application class will be that first class to load.
So it will be easier to store some randomly used values in the application class.
public class Info extends Application
{
public static int ID = 0;
public static String NAME = "TEST";
}
Then call it in any activity by:
Info info= ((YourApplication)this.getApplication());
And in your manifest:
<application
android:name="<your package name>.GlobalApplication">
........
........
</application>

Well, that way doesn't always work in Android. Your static values are hold only while your app is running. Imagine you are sharing some content with action SEND, so you are starting another app which actually share your content (Facebook, email, etc.). Android may decide to stop completely your app if there are no resources enough to launch other app. In that point, the process of your app is completely gone and, with it, your static values. When going back to your app from the app that shared the content, you've lost your values.
I'd better use Intent extras, combined with Parcelable objects if you need to serialize more complex data.
You can easily try it if you enable in a device the option Don't keep activities under developer options, which destroys every activity as soon as the user leaves it.

How to I can call a method from another activity with passing data?

I have 2 activities.
I start one, and at a point I start another one:
Intent enabler = new Intent(this, cprompt.class);
startActivity(enabler);
I want to call a method from the old one, but I want to pass some data too.
This is how I tried:
Activity1.sendcommand(DATA);
And I got this:
Cannot make a static reference to the non-static method sendcommand(String) from the type Activity1
And I don't want change the method to static.
The only way if I make a Listener? If I have to, you can describe for me to how?
The java is new for me... :/ But if this problem is solved I think I am done with my program :)

Only one Activity is active at a time. Are you trying to trigger the method just because you want to pass some data?.You can do it via an Intent
Intent intent = new Intent(this, SecondActivity.class);
intent.putExtra("YOUR_DATA_KEY", "Data Value");
startActivity(intent);
and in the SecondActivity , you can retrieve the data and process it
String data = getIntent().getStringExtra("YOUR_DATA_KEY");
It's not just Strings that you can send this way.You can even transmit Serializable and Parcelable Extras. For more information refer - Intents on Android Developer Website

You can declare an interface and make Activity1 implements that interface..
Then when you need to call Activity1's method call the inetrface's method...
Your interface:
public interface OnSendCommand{
public void onSendCommand(values);
}
Activity1:
public Activity1 implements OnSendCommand {
public void onSendCommand(values){}
}

Are you returning to the first activity from the second when executing this method? If so you could use startActivityForResult() following this tutorial. If not, then something is wrong with the structure of your application if this method can not be static, and this method should be in a separate class not inheriting from Activity.

Android: how can fragment take a global variable of Activity

I have an activity with a global variable int x, how can a fragment get the current value of variable x of its activity ?

Either set the var as public static, or use
((MyActivity)getActivity()).getX()

Using a public static variable isn't the best way to communicate between an activity and a fragment. Check out this answer for other ways:
The Android documentation recommends using an interface when the Fragment wants communicate with the Activity. And when the Activity wants to communicate with the Fragment, the Activity should get a reference to the Fragment (with findFragmentById) and then call the Fragment's public method.
The reason for this is so that fragments are decoupled from the activity they are in. They can be reused in any activity. If you directly access a parent Activity or one of its global variables from within a fragment, you are no longer able to use that fragment in a different Activity.

Kotlin version:
(activity as MyActivity).x

***In your Activity
==================
Bundle args = new Bundle();
args.putInt("something", Whatever you want to pass);
fragA.setArguments(args);
In your Fragment
==================
Bundle args = getArguments();
//whatever you want to get ,get it here.
//for example integer given
int index = args.getInt("index", 0);